This invention relates generally to electrical instrumentation systems that display, via air core instrument gauges, the values of parameters that are monitored by associated sensors. Instrumentation systems of this type are commonly used in automotive vehicles. The air core gauges are mounted in the instrument cluster of a vehicle to display the values of various operating parameters that are of interest, such as engine speed, engine oil temperature, engine oil pressure, etc.
More specifically, the invention relates to such a system in which air core gauges are adapted by means of electronic circuitry for use with a microprocessor that provides data for the gauges in digital form on an address/data bus.
An air core gauge comprises an electromechanical movement that is operated by an input whose value is representative of the particular parameter that is to be indicated by the gauge. As the value of the input to the gauge changes, so does the amount of deflection of the movement, and this produces a corresponding amount of deflection of a needle on the face of the gauge thereby providing an indication of the value of the parameter of interest. Two types of air core gauges that are used in automotive instrumentation systems are known respectively as sine/cosine meters and and sensor-driven meters. A sine/cosine meter receives respective sine and cosine input signals, the respective phases of which are variable to produce corresponding deflection of the gauge needle. In a sensor-driven meter the amount of DC current flowing through the movement is varied to produce a corresponding amount of needle deflection. In general a sine/cosine meter has a greater span of angular travel than does a sensor-driven meter.
Even with the advent of electronic systems, such as digital microprocessor-based ones, air core type gauges continue to enjoy substantial useage. In general such gauges can be mass produced at considerably lower costs than gauges that have digital electronic displays. Moreover, many people prefer the indication of a needle over the presentation of a number.
The present invention is concerned principally with a new and unique means for interfacing air core meters to a microprocessor with the use of minimal circuitry. This is particularly advantageous in automotive and truck instrumentation applications in facilitating the integration of microprocessors into these vehicles. One aspect of the disclosed embodiment of the invention is for adapting air core gauges to use digital data that is sourced from an SAE ATA serial data link so that engine and other subsystem information available on the data link may be displayed on gauges that are operated by a host microprocessor.
In order to accurately present an indication of the value of a particular parameter of interest, an air core gauge requires a continuous input. A further aspect of the present invention relates to the time multiplexed control of air core gauges from the host microprocessor. According to the disclosed implementation of the invention, each instrument gauge is assigned to be a particular output device on the microprocessor's memory map. The microprocessor addresses a particular gauge via the address bus at the same time that data for that gauge is present on the data bus. Electronic circuitry that is associated with each gauge is effective to retain the data and cause the gauge needle to continously indicate the correct needle deflection corresponding to the retained data.
The closest known approach to providing a continuous air core meter movement deflection from data that is intermittently present on a data bus from a microprocessor is disclosed in the commonly assigned co-pending application of Robert Onesti "STAND-ALONE UNIVERSAL GAUGE", Ser. No. 136,223, filing date Dec. 21, 1987. According to the diclosure of the Onesti patent application, a particular combination of electronic circuitry is embodied in an air core gauge to cause the gauge to indicate the latest value of a multi-bit data word supplied to the gauge from the microprocessor via the data bus. The data need be presented to the gauge only intermittently based on how fast the data is capable of changing. Thus, the system of the Onesti patent application is broadly a time multiplexing of individual air core gauges from a microprocessor. According to the Onesti invention, each air core gauge comprises a multi-bit latching memory circuit in which the latest multi-bit of digital data from the microprocessor for the particular gauge is latched. Each gauge further comprises a digital comparator circuit and a multi-bit counting circuit, the latter circuit repeatedly counting a predetermined number of bits at a particular counting frequency. The digital comparator receives the outputs of the digital counting circuit and of the multi-bit latching memory circuit. A flip-flop is operated by the digital comparator circuit and the relative proportion of the flip-flop set time to the flip-flop reset time is indicative of the value of the count that is latched in the gauge's multi-bit latching memory circuit. The output of the flip-flop therefore provides a pulse-width modulated waveform whose degree of pulse width modulation is representative of the value of data latched in the multi-bit latching memory circuit. This pulse width modulated waveform operates the air core gauge movement, producing a deflection corresponding to the value of the data latched in the gauge's multi-bit latching memory circuit. The data is periodically updated so that the needle can follow changes in the data.
The present invention relates to a new and unique organization and arrangement of electronic circuitry that enables an air core gauge to be operated by digital data from a microprocessor that is intermittently presented on a data bus. This electronic circuitry comprises a relatively few number of circuit components for each gauge. In particular, each gauge comprises only a sample and hold circuit, an air core driver circuit and an air core meter. The sample and hold circuits share a common digital-to-analog (D/A) converter which is interposed between the digital data bus, on which digital data periodically appears, and the sample and hold circuits of the individual gauges. The D/A converter converts each piece of data into an analog form which is presented on a common line to the analog sample and hold circuits of all the gauges.
A meter selection logic circuit is also included to receive digital address information from the microprocessor memory map. The particular address that is being supplied on the address bus to the meter selection logic circuit is correlated with the particular data that is present on the data bus. The meter selection logic causes the analog information from the D/A converter to be sampled and held by the particular gauge that is identified by the meter selection logic. In this way, the correct data is supplied to the correct gauge. The sample and hold circuit that is particular to each gauge is capable of holding the information that has been multiplexed to it. It thereby provides a corresponding continous output signal to the gauge's air core driver circuit which in turn drives the air core meter movement to a corresponding deflection so that the meter indicates the value of the data that is in its sample and hold circuit. Each gauge need be updated at a rate corresponding to a rate which will overcome the "droop rate" of the sample and hold being used. Typically ten times a second update rate is sufficient.
One of the attributes of an instrumentation system that embodies principles of the present invention is that there is an opportunity for substantial gauge commonality. This is because data is prescaled in the microprocessor to produce a given meter deflection. In other words, to produce a particular reading on a particular gauge, the value of the data to produce the correct gauge reading is the value of the data that is supplied to the sample and hold circuit that is associated with that gauge. In this way, gauges of common design can be substituted for one another and the same amount of deflection will be assured for the same data signal. With this commonality of gauges it may be unnecessary to procure and stock different types of gauges such as different voltmeters, amperage meters, resistive sensor meters, etc. In obtaining this gauge commonality it is assumed that any legend that is associated with a particular gauge would be either readily changeable or else that such a legend would be incorporated as a fixed part of the instrument cluster design.
The foregoing features, advantages, and benefits, of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a presently preferred embodiment of the invention in accordance with the best mode contemplated at the present time in carrying out the invention.